This invention generally relates to the field of fabrication and use of nanometer-scale metal oxide particles, in particular to a tin (IV) oxide nanopowder consisting of crystalline particles and methods for its preparation and use.
Tin (IV) oxide (SnO2) having a rutile-type crystalline structure is an n-type wide band semiconductor in its undoped form. This material is widely used in a variety of devices because it combines chemical stability in acids and bases, high corrosion resistance, and good electrical conductivity with transparency in the visible optical spectrum. For example, nanocrystalline SnO2 powder is often used as a transparent current collector in electrochromic devices, as a conductive high-temperature ceramics, and in gas sensor applications. Further, this material""s high corrosion resistance coupled with electrical conductivity has aroused considerable interest in using SnO2 as a non-consumable anode in electrolytic production of aluminum. Finally, SnO2 is a promising anode material for use in lithium rechargeable batteries. Electrochemical performance of SnO2, however, greatly depends upon the particle size, and purity of the nanocrystalline powder.
Traditionally, methods of producing pure SnO2 in a powder form were confined to high-temperature hydrolysis of tin (IV) chloride or to the oxidation of gaseous tin (II) oxide at 1300 K. or higher (see, Jarzhebski Z. M., Marton J. P., J. Electrochem. Soc., 123, No.7, 199C (1976)).
Various processes for the preparation of metal oxide powders in general and of tin oxide powder in particular are disclosed in U.S. Pat. No. 6,139,816. In particular, cracking, physical vapor deposition, chemical vapor deposition, spray pyrolysis, gel method, and hydrothermal method have been disclosed. Cracking is simple but cannot provide the desired particle size and distribution because the particles prepared are not uniform. Both physical and chemical vapor depositions have to be conducted under vacuum conditions and require high operation costs. The particles provided by spray pyrolysis are typically too large to be useful in applications. Gel method can provide a desired particle size but is complex and costly because it uses metal alkoxides, which are expensive and easily flammable. Hydrothermal method is a modification of the gel method that avoids some of its shortcomings by using metal salts instead of alkoxides. The hydrothermal method, however, is also expensive because of high-temperature and pressure conditions of the hydrothermal equipment.
There is also a sol-gel method of preparation of nanocrystalline tin oxide particles, disclosed in U.S. Pat. No. 6,395,053. This method is based on the a basic solution, e.g. NH3. Such synthesis of nanocrystalline tin oxide particles leads to a marked increase of mean particle size when treated at temperatures ranging from 450 to 800xc2x0 C.
A process, according to U.S. Pat. No. 6,200,674 includes pyrolyzing a molecular stream consisting of a tin precursor, such as SnCl4, an oxidizing gas, such as oxygen, and a radiation absorbing gas in a reaction chamber. The pyrolysis preferably is driven by heat absorbed from a laser beam, such as a CO2 laser. Thusly obtained tin oxide nanoparticles have an average diameter from about 5 nm to about 100 nm. The reaction conditions determine the properties of the tin oxide particles produced by laser pyrolysis. The appropriate reaction conditions, which should be precisely controlled to produce a certain type of particles, generally depend on the design of a particular apparatus.
Known solution-based and pyrolysis-based methods, such as those described above, share common deficiencies, such as high production costs, and complexity of the equipment involved, as well as presence of amorphous phases of SnO and SnO2, crystalline SnO, and byproducts in the final product. The byproducts typically include residues of a tin precursor, such as tin chlorides and organic or inorganic compounds from the solutions. In addition, post-production calcination of the final product, which is typically necessary to crystallize the amorphous phase and to oxidize SnO into SnO2, results in a uncontrolled growth of individual particles and associated sintering of neighboring particles. Such uneven particle growth may compromise the size uniformity of the nanopowder and may even increase the particle size beyond nanometer scale. Because physical properties of oxides, including SnO2, substantially depend on the degree of deviation from the stoichiometric composition (native disorder) as well as on the type and concentrations of impurities incorporated into the crystalline lattice, the unpredictable nature and amount of contamination and size deviation that is inherent in known processes of tin (IV) oxide nanopowder synthesis lead to variations of product properties that are hardly acceptable for modern technologies. Moreover, known methods frequently require heavy and costly equipment, which complicates their implementation on an industrial scale.
Thus, there remains an unresolved need in the art for an improved method of forming tin (IV) oxide nanoparticles.
It is an object of the present invention to produce tin (IV) oxide crystalline nanometer-scale particles in a powder form (xe2x80x9ctin (IV) oxide nanopowderxe2x80x9d), which are essentially free of byproducts and have reproducible physical properties.
It is another object of the present invention to provide an efficient and inexpensive method of preparation of such tin (IV) oxide nanopowder.
It is yet another object of the present invention to provide a coating including tin (IV) oxide nanopowder with predictable and consistent properties on a wide variety of substrates that is useful in a number of industrial applications.
It is still another object of the present invention to provide a device, for example an electrode, including tin (IV) oxide nanopowder with predictable and consistent properties.
Accordingly, a tin (IV) oxide nanopowder consisting of crystalline particles with rutile crystalline structure and is essentially free of byproducts, is disclosed herein. Also disclosed herein are methods for preparation of such nanopowder that provide for exclusion of byproducts and use of such nanopowder in coatings and various applications.
A key aspect of the present invention involves preparation of a tin (IV) oxide nanopowder that is essentially free of byproducts by an inexpensive process of a chemical reaction of either a tin chloride or tin sulfates in an ionic melt of alkali metal nitrates followed by cooling, leaching with distilled water, and a thermal treatment. The nanopowder exhibits electrical conductivity that is substantially temperature-independent in a wide range of temperatures.
In general, in one aspect, the invention features a tin oxide nanopowder consisting a plurality of tin (IV) oxide crystalline particles, each of this plurality of crystalline particles having rutile crystalline structure, wherein said nanopowder is essentially free of byproducts.
In general, in another aspect, the invention features a method for preparation of a tin oxide nanopowder consisting a plurality of tin (IV) oxide crystalline particles, each of this plurality of crystalline particles having rutile crystalline structure, that includes providing a tin oxide precursor, providing at least one nitrate of an alkali metal, and creating a starting mixture of this tin oxide precursor and this at least one nitrate. The method further entails heating the starting mixture to a temperature effective for conducting a chemical reaction between the tin oxide precursor and the nitrate, and then curing the starting mixture at this temperature for a period of time until the chemical reaction concludes. The method further includes cooling a resulting mixture to an ambient temperature, leaching the resulting mixture with a liquid solvent thereby creating a suspension, and separating the tin (IV) oxide nanopowder from the suspension. The method concludes with heating the tin (IV) oxide nanopowder and curing it to remove residual moisture therefrom.